Piezo pumped heat pipe

ABSTRACT

An arrangement is provided for cooling a heat-generating device (e.g., an integrated circuit chip) in a system such as a laptop computer. The arrangement includes a piezo pumped heat pipe having a piezoelectric device near an evaporator in the heat pipe. The piezoelectric device, when actuated, helps reduce evaporator resistance when the evaporator evaporates a liquid coolant in the heat pipe.

BACKGROUND

1. Field

The present invention relates generally to liquid cooling systems and,more specifically, to heat pipes for dissipating heat generated byintegrated circuits.

2. Description

As integrated circuits (e.g., central processing units (CPUs) in acomputer system) become denser, components inside an integrated circuitchip are drawing more power and thus generate more heat. Various liquidcooling systems have been used to dissipate heat generated by integratedcircuit chips, for example within personal computers, mobile computers,or similar electrical devices. A liquid cooling system circulates aliquid coolant (e.g., water) through a heat sink attached to anintegrated circuit chip inside of a device such as a computer. As theliquid passes through the heat sink, heat is transferred from the hotintegrated circuit chip to the cooler liquid. The hot liquid (or thevapor of the liquid) then moves out to a radiator at the back (or side)of the case of the device and transfers the heat to the ambient airoutside of the case. The cooled liquid then travels back through thesystem to the integrated circuit chip to continue the process.

A heat pipe is a commonly used form of heat sink in a liquid coolingsystem to dissipate heat generated by integrated circuits, especiallyCPUs, inside a computer system. A heat pipe may include an evaporatorsection and a condenser section. Heat may be transferred from theevaporator section to the condenser section through vapor generated byan evaporator in the evaporator section by evaporating a liquid coolant.The vapor may condense back to liquid form at the condenser sectionthrough a heat exchanger coupled to the heat pipe. A heat pipe may alsoinclude a wick to act as a pump to bring the liquid coolant back fromthe condenser section to the evaporator section. The evaporator mayagain evaporate the liquid coolant, drawing to the evaporator section bythe wick, when heated by the heat generated by an integrated circuitchip. The heat transfer rate from the integrated circuit chip into theliquid coolant in the evaporator section depends on evaporationresistance. The lower the evaporation resistance is, the higher the heattransfer rate is. Thus, it is desirable to reduce the evaporationresistance whenever possible.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will becomeapparent from the following detailed description of the presentinvention in which:

FIG. 1 illustrates an exemplary block diagram of a computer system whichmay be utilized to implement embodiments of the present invention;

FIG. 2 illustrates an exemplary block diagram of a piezo pumped heatpipe along with a heat exchanger, according to one embodiment of thepresent invention;

FIG. 3 is a block diagram illustrating an example of a piezo pumped heatpipe, according to one embodiment of the present invention;

FIG. 4 is an internal top view of an example implementation of a piezopumped heat pipe, according to one embodiment of the present invention;and

FIG. 5 is a side view of the piezo pumped heat pipe whose top view isshown in FIG. 4, according to one embodiment of the present invention.

DETAILED DESCRIPTION

Evaporation resistance in a heat pipe may be affected by many factorssuch as evaporator structure and flow velocities of liquid around theevaporator. High flow velocities of liquid around the evaporator canmake the evaporation mechanism in the evaporator more like flow boilingmechanism than like thin film evaporation mechanism. Typically flowboiling mechanism in the evaporator results in lower evaporationresistance than does thin film evaporation mechanism. According to anembodiment of the present invention, a piezoelectric device may be usedto induce flow boiling in the evaporator in a heat pipe. A piezoelectricmaterial can convert between mechanical and electrical energy. Anelectric potential applied to a piezoelectric material causes a smallchange in the shape of the material. Likewise, physical pressure appliedto a piezoelectric material creates an electrical potential differencebetween the surfaces of the material. The piezoelectric device may beembedded near the evaporator in the heat pipe. Upon actuation, thepiezoelectric device may generate mechanical vibrations, which oscillateliquid in the evaporator section. The oscillating motions generated bythe piezoelectric device may increase flow velocities of the liquid inthe evaporator section to generate flow boiling characteristics, andthus reduce evaporation resistance.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present invention means that a particular feature, structure orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrase “in one embodiment” appearing in variousplaces throughout the specification are not necessarily all referring tothe same embodiment.

FIG. 1 illustrates an exemplary block diagram of a computer system whichmay be utilized to implement embodiments of the present invention.Although not shown, the computer system is envisioned to receiveelectrical power from a direct current (DC) source (e.g., a battery)and/or from an alternating current (AC) source (e.g., by connecting toan electrical outlet). The computer system comprises a centralprocessing unit (CPU) or processor 110 coupled to a bus 115. For oneembodiment, the processor 110 may be a processor in the Pentium® familyof processors including, for example, Pentium® 4 processors, Intel'sXScale® processor, Intel's Pentium® M processors, etc., available fromIntel Corporation. Alternatively, other processors from othermanufacturers may also be used.

The computer system as shown in FIG. 1 may also include a chipset 120coupled to the bus 115. The chipset 120 may include a memory control hub(MCH) 130 and an input/output control hub (ICH) 140. The MCH 130 mayinclude a memory controller 132 that is coupled to a main memory 150.The main memory 150 may store data and sequences of instructions thatare executed by the processor 110 or any other device included in thesystem. For one embodiment, the main memory 150 may include one or moreof dynamic random access memory (DRAM), read-only memory (RAM), FLASHmemory, etc. The MCH 130 may also include a graphics interface 134coupled to a graphics accelerator 160. The graphics interface 134 may becoupled to the graphics accelerator 160 via an accelerated graphics port(AGP) that operates according to an AGP Specification Revision 2.0interface developed by the Intel Corporation. A display (not shown) maybe coupled to the graphics interface 134.

The MCH 130 may be coupled to the ICH 140 via a hub interface. The ICH140 provides an interface to input/output (I/O) devices within thecomputer system. The ICH 140 may be coupled to a Peripheral ComponentInterconnect (PCI) bus. The ICH 140 may include a PCI bridge 145 thatprovides an interface to a PCI bus 170. The PCI Bridge 145 may provide adata path between the CPU 110 and peripheral devices such as, forexample, an audio device 180 and a disk drive 190. Although not shown,other devices may also be coupled to the PCI bus 170 and the ICH 140.

The CPU 110, the chipset 120, and other devices in the computer systemas shown in FIG. 1 may use a piezo pumped heat pipe to dissipate heatgenerated by them.

FIG. 2 illustrates an exemplary block diagram of a piezo pumped heatpipe along with a heat exchanger, according to one embodiment of thepresent invention. The piezo pumped heat pipe 210 comprises a sealedcontainer whose inner surfaces have a capillary material that forms thewick (not shown in FIG. 2). One end (212) of the piezo pumped heat pipemay be coupled to a heat-generating device 230 (e.g., a processor). Theheat generated by the device 230 transfers to the working fluid insidethe heat pipe by evaporating the working fluid. The section inside theheat pipe (near the heat-generating device) where the working fluid isevaporated is also called an evaporator section. The pressure differenceinside the heat pipe may help transport the vapor of the working fluidfrom the evaporator section to the other end of the heat pipe, which maybe coupled to a heat exchanger 240. The heat exchanger 240 transfers theheat from the vapor to ambient air so that the vapor may condense backto liquid. The section inside the heat pipe (near the heat exchanger)where the vapor condenses is also called a condenser section. After thevapor condenses, the liquid then moves to the evaporator section withthe help of the wick. This process continues so long as theheat-generating device generates enough heat to evaporate the workingfluid in the evaporator section.

The container is leak-proof so that it can isolate the inside workingfluid from the outside environment. The container maintains the pressuredifferential across its walls, and enables transfer of heat to takeplace from and into the working fluid. Selection of the containermaterial depends on many factors such as compatibility (both withworking fluid and external environment), strength to weight ratio,thermal conductivity, ease of fabrication, and porosity. The materialshould be non-porous to prevent the diffusion of the vapor of theworking fluid. A high thermal conductivity ensures minimum temperaturedrop between the heat source and the wick. Although it is shown as arectangular “L” shape in FIG. 2, the container can be any other shape(e.g., a straight or “L” shape cylinder) and any size so long as the end212 can be made fit with a heat-generating device and the other end canbe made fit to a heat exchanger.

It is desirable that the working fluid can be evaporated by aheat-generating device. In one embodiment, the working fluid may bewater, alcohol, glycol, an inert liquid, combinations thereof,surfactants, mixtures thereof, and the like. A high value of surfacetension may be desirable in order to enable the heat pipe to operateagainst gravity and to generate a high capillary driving force. Inaddition to high surface tension, it is also desirable for the workingfluid to wet the wick and the container material. A high latent heat ofvaporization is desirable in order to transfer large amounts of heatwith minimum fluid flow, and hence to maintain low pressure drops withinthe heat pipe. The thermal conductivity of the working fluid shouldpreferably be high in order to minimize the radial temperature gradientand to reduce the possibility of nucleate boiling at the wick or wallsurface. The resistance to fluid flow will be minimized by choosingfluids with low values of vapor and liquid viscosities.

The capillary structure or the wick over the inner surfaces (not shownin FIG. 2) of the container may be a porous structure made of materialslike steel, aluminum, nickel or copper in various ranges of pore sizes,fabricated using metal foams. Fibrous materials, such as ceramics andcarbon fiber filaments, may also be used. The main purpose of the wickis to generate capillary pressure to transport the working fluid fromthe condenser section to the evaporator section. It should also be ableto distribute the liquid around the evaporator section to any area whereheat is likely to be received by the heat pipe. The selection of thewick for a heat pipe depends on many factors. The maximum capillary headgenerated by a wick increases with decrease in pore size. The wickpermeability increases with increasing pore size. Another feature of thewick is its thickness. The heat transport capability of the heat pipemay be raised by increasing the wick thickness. The overall thermalresistance in the evaporator section also depends on the conductivity ofthe working fluid in the wick. Other necessary properties of the wickare compatibility with the working fluid and wettability. Types ofcommonly used wick comprises sintered powder, grooved tube, and screenmesh.

Although it is desirable that the working liquid can be evaporated bythe heat from a heat-generating device, in one embodiment, there may beno evaporation process or only a partial evaporation process. The colderliquid may move from one end, which is coupled to a heat exchanger tothe other end, which is coupled to a heat-generating device, and isheated there to become hotter liquid (or hotter liquid and vapormixture), which then moves back to the colder end.

A piezoelectric device (not shown in FIG. 2) may be placed at the end212 near the evaporator section. The piezoelectric device may beactuated by an oscillating voltage source (not shown in FIG. 2) viawires 220. When actuated, the piezoelectric device may generateoscillating or wavy motions in the liquid in the evaporator section toincrease flow velocities and thus generate flow boiling characteristics.The flow boiling characteristics may reduce evaporation resistance andincrease the efficiency of heat transfer from the heat-generating deviceto the liquid. The heat generated by the heat-generating device causesthe liquid in the evaporator section to evaporate and enter a vaporstate. The vapor, which has a higher specific volume, moves inside thesealed container to the other end of the heat pipe that is coupled tothe heat exchanger 240. The heat exchanger 240 may include a fan 242 toprovide higher air flow. Heated air 250 may be rejected by the fan 242into the ambient air. When the vapor condenses to liquid at the heatexchanger end of the heat pipe, it transfers the heat to the heatexchanger walls. The heat exchanger walls may further transfer energy toambient air with the help of the heat exchanger 240. The liquid thenmoves back to the evaporator section through the wick.

FIG. 3 is a block diagram illustrating an example of a piezo pumped heatpipe, according to one embodiment of the present invention. An attachblock 310 may attach the heat pipe 340 to a heat-generating device 330.In one embodiment, the attach block may be a part of the heat pipe asshown in FIG. 2. In another embodiment, the attach block may be coupledto the heat pipe so that the attach block may efficiently transfer heatfrom the heat-generating device 330 to the evaporator section of theheat pipe 340. For example, when the container of the heat pipe is acylinder and a heat-generating device has a flat surface, an attachblock may be used to attach the heat pipe to the heat-generating device.Alternatively, the cylinder-shaped heat pipe may be made to have a flatend to serve as the attach block. The attach block 310 may comprise apiezoelectric device 315. The piezoelectric device is located inside theheat pipe 340 near the evaporator section (in the case that the attachblock is separate from the heat pipe, the piezoelectric device 315 is inthe heat pipe). The actuating device 320 applies an oscillating voltageto the piezoelectric device so that the piezoelectric device cangenerate wavy motions. In one embodiment, the actuating source may belocated outside the heat pipe. In another embodiment, the actuatingsource may be located inside the heat pipe.

Inside the heat pipe 340 there may be a vapor area 342 and a wick 344.The vapor generated in the evaporator section may transport through thevapor area 342 to the colder end of the heat pipe because of pressuredifference between the colder end and the hotter end where theevaporator section locates. Opposite to the end where the piezoelectricdevice is located, the other end of the heat pipe may be coupled to aheat exchanger 350. The heat exchanger may comprise a fan 352 and aplurality of fins 354. The fan 352 helps increase air circulation togenerate higher air flow so that heat carried by the vapor inside theheat pipe may be dissipated faster. The plurality of fins 354 increasethe contact area between the heat exchanger and the ambient air toimprove efficiency of heat transfer from the vapor inside the heat pipeto the ambient air. When the vapor transfers heat inside it to theambient air through the heat exchanger, the vapor condenses and returnsto the liquid state. The liquid then moves back to the evaporatorsection (not shown in FIG. 3) through capillary actions of the wick 344.

FIG. 4 is an internal top view of an example implementation of a piezopumped heat pipe, according to one embodiment of the present invention.The piezo pumped heat pipe may include outer walls 410, a wick 420, anevaporator 430, and a piezoelectric device 440. Although not explicitlyillustrated in FIG. 4, the piezo pumped heat pipe may also include aliquid coolant. An evaporator section of the heat pipe of may be locatednear evaporator 430, and a condenser section of heat pipe may be spacedapart from the evaporator (e.g., including the far end of heat pipe).The piezo pumped heat pipe may be of any size. Outer walls 410 mayenclose wick 420, evaporator 430, piezoelectric device 440, and thecoolant. Outer walls 410 may be coupled to a heat-generating device(e.g., an integrated circuit chip), and they may include a highlythermally conductive material, such as copper or another material. Outerwalls 410 may be formed in a roughly rectangular shape, as illustratedin FIG. 1, or any other geometry that facilitates access to evaporator430 by the coolant and facilitates contact between outer walls andsurfaces of the heat-generating device. Outer walls may also be formedto prevent the escape of vapor or liquid.

Wick 420 may include a porous material (e.g., sintered spherical copperparticles, sintered metal powder, a fiber material, and/or a screenmaterial), or a porous material with a grooved surface, which covers aninner surface of the piezo pumped heat pipe, except for the areaoccupied by evaporator 430. Wick 420 may, by virtue of its porousstructure, bring coolant from the condenser section of heat pipe to theevaporator section at or near evaporator 430. In this manner, wick 420may act to hydrate evaporator 430. In other implementations, wick 420may include axial grooves that act to bring coolant from the condensersection of heat pipe to the evaporator section. Other types ofhomogenous structures for wick 420 may include an open annularstructure, an open artery structure, and/or an integral arterystructure. In still other implementations, various composite structuresmay be used for wick 420 that may include one or more of the homogeneousstructures noted above (e.g., sintered particles, screen, fibers,grooves, etc.). Wick 420 may be designed to have a relatively highcapillary pumping efficiency to hydrate evaporator 430.

Evaporator 430 may include a porous material (e.g., spherical metalparticles of various sizes sintered onto the inner surface of outer wall410) that roughly corresponds in area and orientation to a surface ofthe heat-generating device to be cooled. In one embodiment, the porousmaterial used for evaporator 430 may be the same as the porous materialused for wick 420. In another embodiment, the evaporator may usedifferent porous material from that used in the wick. The porousmaterial of evaporator 430 may include, for example, copper particles.In one embodiment, the evaporator may include a grooved surface. Thegrooved surface may be made of the same material as the container of theheat pipe.

The piezoelectric device 440 may be located near the evaporator. Whenactuated by an oscillating voltage source, the piezoelectric devicegenerates wavy motions in the liquid in the evaporator section. Theliquid is brought to the evaporator section from the condenser sectionof the heat pipe by capillary pumping of the wick. Without thepiezoelectric device 440, the liquid flow in the evaporator section isdriven by capillary actions and flow velocities of the liquid in theevaporator section may be smaller than those with wavy motions generatedby piezoelectric device 440. As a result, evaporation resistance may behigher without piezoelectric device 440. Evaporation resistance dependson the evaporation/boiling process in the evaporator section of the heatpipe. Lower evaporation resistance may result in higher heat transferefficiency for the heat pipe. Typically, a thin film evaporation processresults in higher evaporation resistance than a flow boiling process forthe same heat flux. Without the piezoelectric device, the boilingprocess in the evaporator section resembles thin film evaporation heattransfer. Wavy motions generated by piezoelectric device 440 may enhancepumping of liquid into the evaporator section. The wavy motions in theliquid in the evaporator section may result in high local velocities inthe liquid. The high local velocities in turn make the boiling processsimilar to the flow boiling process. Therefore, piezoelectric device 440may help reduce evaporation resistance and thus increase heat transferefficiency.

FIG. 5 is a side view of the piezo pumped heat pipe whose top view isshown in FIG. 4. The heat pipe shown in FIGS. 4 and 5 has anapproximately rectangular cross-sectional shape. In addition to outerwalls 410, wick 420, evaporator 430, and piezoelectric device 440, whichare discussed above with regard to FIG. 4, the piezo pumped heat pipemay also include a liquid coolant 510 and a vapor space 520. The liquidcoolant 510 may include water, methanol, ethanol, acetone, heptane,Freon, or another refrigerant, or a mixture of two or more types ofliquids. The liquid coolant may pool on the surface of the evaporator,as illustrated in FIG. 2, and may also permeate wick 420. The liquidcoolant may be evaporated by boiling over the evaporator. In oneembodiment, wick 420 may extend vertically above the evaporator toimprove wetting of evaporator by the liquid coolant. In anotherembodiment, wick 420 may not extend vertically above the evaporator. Insuch an embodiment, however, it is desirable that the amount of coolant510 be sufficient to ensure continuous wetting of the evaporator. Ineither embodiment, wavy motions generated by piezoelectric device 440may improve wetting of the evaporator.

Vapor space 520 may be located between wick 420 and the top one of outerwalls 410. When liquid coolant 510 is evaporated by boiling over theevaporator in the evaporator section, the vapor pressure in theevaporator section becomes higher than that in the condenser section.The pressure difference thus helps transport vapor to the condensersection of the peat pipe via vapor space 520 (and possibly also wick420), where it cools, becomes liquid, and is transported back to theevaporator section by the wick.

Although an example embodiment of the present disclosure is describedwith reference to diagrams in FIGS. 1-5, persons of ordinary skill inthe art will readily appreciate that many other methods of implementingthe present invention may alternatively be used. For example, the orderof execution of the functional blocks or process procedures may bechanged, and/or some of the functional blocks or process proceduresdescribed may be changed, eliminated, or combined.

In the preceding description, various aspects of the present disclosurehave been described. For purposes of explanation, specific numbers,systems and configurations were set forth in order to provide a thoroughunderstanding of the present disclosure. However, it is apparent to oneskilled in the art having the benefit of this disclosure that thepresent disclosure may be practiced without the specific details. Inother instances, well-known features, components, or modules wereomitted, simplified, combined, or split in order not to obscure thepresent disclosure.

While this disclosure has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications of the illustrative embodiments,as well as other embodiments of the disclosure, which are apparent topersons skilled in the art to which the disclosure pertains are deemedto lie within the spirit and scope of the disclosure.

1. A cooling apparatus, comprising: a piezo pumped heat pipe to transferheat from a first section to a second section; and a heat exchanger,coupled to the piezo pumped heat pipe, to dissipate heat from the secondsection of the piezo pumped heat pipe.
 2. The cooling apparatus of claim1, wherein the first section corresponds to an evaporator section in thepiezo pumped heat pipe, and the second section corresponds to acondenser section in the piezo pumped heat pipe.
 3. The coolingapparatus of claim 1, wherein the piezo pumped heat pipe comprises: aliquid coolant; an evaporator, when heated, to evaporate at least aportion of the liquid coolant; a wick structure to bring the at least aportion of the liquid coolant to the evaporator; and a piezoelectricdevice adjacent to the evaporator to help reduce evaporator resistancewhen the evaporator evaporates the at least a portion of the liquidcoolant.
 4. The cooling apparatus of claim 3, wherein the piezo pumpedheat pipe further comprises a container to enclose the evaporator, thewick structure, the liquid coolant, and the piezoelectric device.
 5. Thecooling apparatus of claim 3, wherein the piezoelectric device isactuated by an oscillating voltage source.
 6. The cooling apparatus ofclaim 5, wherein the piezoelectric device, when actuated, generates wavymotions in at least a portion of the liquid coolant, the wavy motionscapable of increasing flow velocities in the liquid coolant near theevaporator.
 7. The cooling apparatus of claim 1, further comprising anoscillating voltage source to actuate a piezoelectric device in thepiezo pumped heat pipe.
 8. The cooling apparatus of claim 1, wherein theheat exchanger extracts heat from vapor in the second section tocondense the vapor to liquid.
 9. The cooling apparatus of claim 8,wherein the heat exchanger comprises a fan to enhance air flow and aplurality of fins to dissipate heat extracted from the vapor to ambientair.
 10. A piezo pumped heat pipe, comprising: a liquid coolant; anevaporator, when heated, to evaporate at least a portion of the liquidcoolant; and a piezoelectric device adjacent to the evaporator to helpreduce evaporator resistance when the evaporator evaporates the at leasta portion of the liquid coolant.
 11. The piezo pumped heat pipe of claim10, further comprising a wick structure to bring at least a portion ofthe liquid coolant to the evaporator.
 12. The piezo pumped heat pipe ofclaim 10, further comprising a container to enclose the evaporator, theliquid coolant, the piezoelectric device, and a wick structure.
 13. Thepiezo pumped heat pipe of claim 10, wherein the piezoelectric device isactuated by an oscillating voltage source.
 14. The piezo pumped heatpipe of claim 13, wherein the piezoelectric device, when actuated,generates wavy motions in at least a portion of the liquid coolant, thewavy motions capable of increasing flow velocities in the liquid coolantnear the evaporator.
 15. A system, comprising: a heat-generating device;and a cooling system to dissipate heat generated by the heat-generatingdevice using a piezo pumped heat pipe.
 16. The system of claim 15,wherein the heat-generating device comprises an integrated circuit chip.17. The system of claim 15, wherein the cooling system comprises: apiezo pumped heat pipe, including an evaporator section and a condensersection, to transfer heat from the evaporator section to a condensersection; a heat exchanger, coupled to the piezo pumped heat pipe, todissipate heat from the condenser section; and an oscillating voltagesource to actuate a piezoelectric device in the piezo pumped heat pipe.18. The system of claim 17, wherein the piezo pumped heat pipecomprises: a liquid coolant; an evaporator, when heated, to evaporate atleast a portion of the liquid coolant; a wick structure to bring the atleast a portion of the liquid coolant to the evaporator; and thepiezoelectric device adjacent to the evaporator to help reduceevaporator resistance when the evaporator evaporates the at least aportion of the liquid coolant.
 19. The system of claim 18, wherein thepiezo pumped heat pipe further comprises a container to enclose theevaporator, the wick structure, the liquid coolant, and thepiezoelectric device.
 20. The system of claim 18, wherein thepiezoelectric device, when actuated, generates wavy motions in at leasta portion of the liquid coolant, the wavy motions capable of increasingflow velocities in the liquid coolant near the evaporator.
 21. Thesystem of claim 17, wherein the heat exchanger extracts heat from vaporin the condenser section to condense the vapor to liquid.
 22. The systemof claim 21, wherein the heat exchanger comprises a fan to enhance airflow and a plurality of fins to dissipate heat extracted from the vaporto ambient air.